>>It seems most of the homebrew and Handbook amps I see are
>>Pi tanks. Is the Pi-L really necessary?<
> To meet the current requirements of the FCC and the Radio
> Regulations on spurious emissions, yes. The Class AB stage
> produces a harmonic content in the plate current that is 6
> dB down on the second harmonic, and if you do the sums,
> getting down the extrs 37dB to meet FCC or 44dB to meet
> the Radio Regs limits is pretty darn difficult in just a
> pi network- in fact, I reckon impossible, although there
> may be somebody who can come up with values that in a
> particular case manage to do it, at least in theory. It
> can even be a bit difficult in a Pi-L under some
> circumstances.
> However, using a pi and an external low pass filter
> selected by band, or a tuner, is another approach
Peter,
Let's not leap to conclusions! The answer is actually no.
Most amplifiers will meet the regulations easily without a
pi-L, and many pi-L's actually are insignificantly better on
harmonics or no better at all.
That whole idea of -6dB harmonic level falls apart because
the -6dB available isn't actually a real solid number. Not
only that, the conclusion based on being -6dB plus the
attenuation of the additional network assumes a matched
coupling on the harmonic, not a mismatched load like the
tank presents. Also the components in the tank are never
perfect linear reactance changes with frequency so you can't
even use the model of the tank. The source feeding the tank
isn't a pure resistance with the same Z as the match on the
fundamental frequency.
The wheels just totally fall off your idea in the real
world.
Typical measurements of AB2 GG single ended amps show the
second harmonic, with a tank Q of only 10-12, is 50dB or
more down on the second harmonic. If you don't meet the spec
of -45db or so, odds are a pi L isn't going to do it either.
As a matter of fact in the real world of antennas presenting
other than 50 ohm loads on the output port of the PA the L
section can just as likely INCREASE harmonic level, not
decrease it.
The layout of the tank and even the tuned input has more to
do with harmonics than tank Q is most cases.
73 Tom
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